This invention relates generally to torque balanced cables, and more particularly to an improved cable useful for towing, suspending, or tethering oceanographic or benthic apparatus or instrumentation.
In the field of oceanography, for example, there are data gathering situations wherein it is desirable to tow or suspend instrumentation at great depths, often measured in hundreds or thousands of meters, and wherein it is advantageous to minimize rotational or angular displacement of instruement stations due to twisting moments or torque developed within the associated cable with changes in loading along the length thereof. In addition, there exists a need for such torque-balanced cables that include signal transmission conductor means such as electrical or fiberoptic conductor means and are strong, lightweight, subject to minimum stretch to assure accuracy of station locations, and sufficiently flexible and durable to permit running over sheaves or around drums during deployment and retrieval.
The development of certain high strength synthetic plastic fiber materials, such as the polyamides, having low coefficients of stretch compared to other synthetic fibers, such the polyethelynes and polyesters, has offered a new dimension in synthetic cable or rope construction. Typical of the polyamide fibers are those sold under the trade name "KEVLAR" by and include "KEVLAR 29" and "KEVLAR 49" having stretch or breaking elongation coefficients of about 3.5% and 2.5%, respectively. One of the greater disadvantages of the polyamide fibers, and one which has presented considerable problems in achieving both flexibility and durability in a cable, is the relatively low resistance to abraision. In this regard "KEVLAR 29" has the better abraision resistance.
Various types of torque free or balanced cables have evolved over the years, each with particular drawbacks. The principal types are braided, parallel fiber, and contrahelically wound. Braided cables are inherently torque balanced but do not exhibit the bending lifetime performance of twisted or contrahelical cables. Parallel fiber cables are torque free but cannot be worked over sheaves. Contrahelically wound cables are aimed at taking advantage of the usually excellent working characteristics of twisted cables, but have not met with good success because of difficulties in obtaining torque balance under differing loads, principally because of differences in the pitch diameters of the strand layers, friction and abrasion between strands and layers, and mismatch of the numbers of strands between layers which will produce the desired balance.